var stream=require("stream");
var MyReadable=stream.Readable;
var rs=new MyReadable();
var c='a'.charCodeAt(0);
rs._read=function(){
    if(c<='z'.charCodeAt(0)){
        rs.push(String.fromCharCode(c++));
    }
    else{
        rs.push(null);
    }
}
rs.pipe(process.stdout);